The rules supplement never says that teleport makes the target an illegal target for future attacks. It states that it cancels the attack because the target moved DURING the attack. This could apply if you are going to make the case that the attack actually begins with the trigger, rather than with the Declare Attack Step, but that's not how the rules seem to be written from my point of view. They seem to clearly state that all attacks begin with the Declare Attack Step and this is reiterated in several places in the rules and rules supplement.
Otherwise, if you are going to say that a teleport which occurs before an attack begins somehow makes the creature an illegal target for all further attacks, we would end up with a whole lot of completely invulnerable creatures running around for the entire game, making it impossible to ever hit them and win the game. Teleport your mage right at the beginning of the game, and you are now immune from ever receiving an attack because you teleported beforehand, making you an illegal target. That's just ludicrous.
Teleport can only affect an attack if it occurs during the attack. Anything else is insanity. The rules state that attacks begin with the Declare Attack Step, not the trigger for the attack. If I have missed something that says otherwise, please point it out to me.
Edit: And in the case of your Block example, if teleport was to cancel the attack, the rules on cancel states it ends immediately. Thus the attack would end BEFORE the Declare Attack Step even occurs, by your logic, so you definitely never make it to the Avoid Attack Step to have the block affected in any way.